In the optical imaging of objects, it is often desirable to generate what is known as a phase contrast image. In the phase contrast image, at least part of the image contrast is caused by a phase shift of the light through the object depicted. This allows in particular objects that bring about no attenuation or only a slight attenuation of the amplitude of the light, but a significant phase shift (phase objects) to be depicted with comparatively higher contrast. Typically, biological specimens as an object in a microscope may bring about a greater change in phase than a change in amplitude of the electromagnetic field.
Various techniques for phase contrast imaging are known, for instance darkfield illumination, oblique illumination, differential interference contrast (DIC) or Zernike phase contrast. Further techniques would be for example the schlieren (knife-edge) method or helical phase contrast.
Such aforementioned techniques have various disadvantages or restrictions. For instance, in the case of the DIC technique, the Zernike technique, the schlieren method and the helical phase contrast, it is typically necessary in comparison with conventional amplitude imaging to provide additional optical elements between the specimen and the detector in the region of what is known as the detection optics. This may lead to restrictions in the structural design, in particular in the case of microscopes of a modular construction. Costs are typically increased. In the case of thin specimens, with darkfield illumination typically only few photons contribute to producing the image, which may lead to noisy images of inferior quality. A subsequent evaluation or analysis of the images may not be possible, or only to a restricted extent. Oblique illumination typically leads to an unsymmetrical increase in contrast, which in turn may bring about reduced quality of the images.